This invention relates to an antenna device for use in mobile radio communication and, in particular, to a multiband antenna capable of performing transmission and reception in a plurality of different frequency bands, and to a multiband portable radio apparatus using the multiband antenna.
Generally, a single antenna device is operable in a single frequency band. To use a radio apparatus in different frequency bands, the radio apparatus is generally required to have a plurality of antenna devices. A typical example is an FM/AM radio receiver.
On the other hand, there is known a trap antenna which is operable over a plurality of separate frequency bands. The trap antenna is often used in amateur radio communication as a multiband antenna.
A conventional trap antenna is disclosed in, for example, Japanese Unexamined Patent Publication (A2) No. 5-121924 (121924/1993).
The conventional trap antenna comprises two strip antenna elements and a resonant circuit or a trap circuit interposed therebetween. The resonant circuit comprises an inductance element (L) and a capacitance element (C) connected in parallel and is referred to as an LC parallel resonant circuit. The LC parallel resonant circuit used in the conventional trap antenna is of a lumped constant type.
However, the conventional trap antenna inevitably has a floating capacitance upon loading the trap circuit. This results in a difference between a theoretical resonant frequency and an actual or measured resonant frequency.
The conventional trap antenna also encounters another problem. Specifically, the trap circuit comprises a capacitor and a coil as the capacitance element and the inductance element, respectively. In addition, a substrate and a shield case are required to support and to shield the capacitor and the coil, respectively. Thus, the conventional trap antenna requires a number of components and assembling steps, and inevitably becomes large in size even though each individual component is small.
In the case where the conventional trap antenna with the above-mentioned structure is used as an external antenna of a radio apparatus, the external antenna is insufficient in strength because of inclusion of the trap circuit comprising the coil and the capacitor. When the radio apparatus is subjected to a mechanical shock, the external antenna is susceptible to damage. Such a disadvantage can result in a serious problem particularly in the case of a portable apparatus.